This invention relates generally to optically variable pigments, films, devices, and images, and more particularly to aligning or orienting magnetic flakes, such as during a painting or printing process, to obtain an illusive optical effect.
Optically variable devices are used in a wide variety of applications, both decorative and utilitarian, for example such devices are used as security devices on commercial products. Optically variable devices can be made in numerous ways to achieve a variety of effects. Examples of optically variable devices include the holograms imprinted on credit cards and authentic software documentation, colour-shifting images printed on banknotes, and enhancing the surface appearance of items such as motorcycle helmets and wheel covers.
Optically variable devices can be made as film or foil that is pressed, stamped, glued, or otherwise attached to an object, and can also be made using optically variable pigments. One type of optically variable pigment is commonly called a colour-shifting pigment because the apparent colour of images appropriately printed with such pigments changes as the angle of view and/or illumination is tilted. A common example is the “20” printed with colour-shifting pigment in the lower right-hand corner of a U.S. twenty-dollar bill, which serves as an anti-counterfeiting device.
Some anti-counterfeiting devices are covert, while others are intended to be noticed. Unfortunately, some optically variable devices that are intended to be noticed are not widely known because the optically variable aspect of the device is not sufficiently dramatic. For example, the colour shift of an image printed with colour-shifting pigment might not be noticed under uniform fluorescent ceiling lights, but more noticeable in direct sunlight or under single-point illumination. This can make it easier for a counterfeiter to pass counterfeit notes without the optically variable feature because the recipient might not be aware of the optically variable feature, or because the counterfeit note might look substantially similar to the authentic note under certain conditions.
Optically variable devices can also be made with magnetic pigments that are aligned with a magnetic field after applying the pigment (typically in a carrier such as an ink vehicle or a paint vehicle) to a surface. However, painting with magnetic pigments has been used mostly for decorative purposes. For example, use of magnetic pigments has been described to produce painted cover wheels having a decorative feature that appears as a three-dimensional shape. A pattern was formed on the painted product by applying a magnetic field to the product while the paint medium still was in a liquid state. The paint medium had dispersed magnetic non-spherical particles that aligned along the magnetic field lines. The field had two regions. The first region contained lines of a magnetic force that were oriented parallel to the surface and arranged in a shape of a desired pattern. The second region contained lines that were non-parallel to the surface of the painted product and arranged around the pattern. To form the pattern, permanent magnets or electromagnets with the shape corresponding to the shape of desired pattern were located underneath the painted product to orient in the magnetic field non-spherical magnetic particles dispersed in the paint while the paint was still wet. When the paint dried, the pattern was visible on the surface of the painted product as the light rays incident on the paint layer were influenced differently by the oriented magnetic particles.
Similarly, a process for producing a pattern of flaked magnetic particles in fluoropolymer matrix has been described. After coating a product with a composition in liquid form, a magnet with a magnetic field having a desirable shape was placed on the underside of the substrate. Magnetic flakes dispersed in a liquid organic medium orient themselves parallel to the magnetic field lines, tilting from the original planar orientation. This tilt varied from perpendicular to the surface of a substrate to the original orientation, which included flakes essentially parallel to the surface of the product. The planar oriented flakes reflected incident light back to the viewer, while the reoriented flakes did not, providing the appearance of a three dimensional pattern in the coating.
It is an object of the present invention, to provide optical variable images wherein three-dimensional objects such as hemi-spheres, cones and the like form images of security devices, labels and the like, using magnetically alignable flakes in the presence of particular magnetic fields, not heretofore described.